Tuning with jets, Power valves and an O2 guage

I pulled the 850 and bolted on a Holley 750 from one of my other cars. The idle vacuum went up both in park and in gear.
The Air/Fuel readings were better as well.
Idle @ 1100 in park : 13.8 to 14.2
Idle @ 850 in gear: 14.2 to 14.6
Pwr brake: 13.8 to 14.5
This shows that the 750 actually stays within tuning range while under a load. The 850 went totally lean when a load was applied.
I wonder if there is something lodged in a passage inside the 850?